1. Field of the Invention
This invention relates to magnetic recording, and in particular to a dual magnetoresistive head (DMR) for reproducing short wavelength, very narrow trackwidth digital data.
2. Description Relative to the Prior Art
The trend in digital disk recording systems continues in the direction of higher areal recording densities. This trend provides increased efficiency in the use of the recording medium, and contributes to the overall compactness and reduction in size of current computers. At the present time, linear recording densities of 50 kilo flux changes per inch and track densities of 2500 tracks per inch result in readily available areal densities of 125 megabits per square inch.
An important component in attaining higher areal densities has been the MR reproduce head utilized in conjunction with a inductive record head. Both shielded and unshielded single element MR heads are known in the art, and each configuration has its advantages and its limitations. However, both configurations provide advantages which include independence of output level on head/media velocity and optimization of the write and read functions of the combined inductive write/ MR read transducer.
The dual MR head (DMR) having a pair of unshielded MR elements as disclosed in U.S. Pat. No. 5,084,794 provides the additional advantages of ease of head fabrication, excellent short wavelength response, a self biasing characteristic, and freedom from MR element shorting problems present in other biased MR head configurations. U.S. Pat. No. 5,084,794 is hereby incorporated by reference.
Referring to FIG. 1, a typical DMR 10 known in the prior art consists of two magnetically, electrically and geometrically matched MR elements 12,14. In this DMR embodiment, the MR elements 12,14 are separated over substantially their entire lengths by an electrically conductive spacer 16. A sense and biasing current 22 flowing to the head 10 via leads 24,26 divides between the two MR elements 12,14 into two equal currents 28,30 because the MR elements 12,14 are identical; a small portion of the current 22 flows in the spacer 16. The DMR 10 is shown in contact with a magnetic medium 11.
The thin film MR elements 12,14 are rectangular in shape. This configuration results in the shape anisotropy of the MR film being directed along the longitudinal axis of the film, which is also the direction of its unbiased magnetization. Static bias fields generated by the flow of sense current rotate the magnetization from this axial direction, and the signals from the magnetic medium further modulate the angular position of the magnetization, changing the film resistance. The currents 28,30 flowing in the same direction in the MR elements 12,14 generate magnetic fields that result in the mutual biasing of the MR elements 12,14 because each of the MR elements, as well as being a field detection element, acts as a soft adjacent biasing layer for the other. The bias field He at element 12 due to this self-biasing mechanism, is equal in magnitude and opposite in sign to the bias field -H.sub.B at the element 14. In biasing the MR elements 12,14 the magnetic field H.sub.B rotates the magnetization of the MR element 12 in one direction, and the field -H.sub.B rotates the magnetization of the MR element 14 in the opposite direction. It will be noted that the biasing fields H.sub.B and -H.sub.B due to the sense current have essentially no longitudinal components; they solely lie along the height directions of the MR element 12,14.
The MR and DMR heads of the prior art have usually been characterized by track widths equal to or greater than 10 times the MR element height. For example, in the above referenced patent the disclosed DMR has a track width of 50 microns and an MR element height of 5 microns. This large ratio of longitudinal extension to height in the heads of the prior art means that operation of the head is essentially "two dimensional" in nature. That is, characteristics of the head operation are determined in the plane perpendicular to that of the longitudinal axis of the MR element, and neither the head sensitivity nor the signal fields from the recorded medium vary appreciably in the longitudinal direction over the active area of the MR element.
For an understanding of the operation of the narrow track width DMR for use in short wavelength signal reproduction, it is advantageous to consider the general signal response mechanism of the MR elements. There are essentially two mechanisms for generation of the MR reproduce signal; 1) direct signal field coupling, and 2) surface-field coupling plus flux propagation. The former accounts for the circumstance when the average (that is, over the MR element active volume) magnetic field emanating from the recorded medium is sufficiently large to directly induce modulation of the MR element magnetization to be observable as magnetoresistive signal. At high ( or even at moderate) linear recording densities, the fields from the medium die off extremely rapidly over distances of the order of typical element heights, and the MR response is due predominately, if not exclusively, to the second mechanism. In this case, the fields from the medium, being essentially non-negligible only at the medium's surface, enter the MR element at the head/medium interface and this surface flux is then internally propagated up into and through the remainder of the MR element. It is this second mechanism which primarily determines the response of the DMR of the present invention to high recording density signals.